190 COPPER-BEARING ROCKS OF LAKE SUPERIOR. 
The conglomerates of Portage Lake differ from each other but little, if at all, in litho- 
logical characteristics. The pebbles vary from the size of a pea to one foot or more in 
diameter, being coarser in some beds than in others. * * * * The pebbles, in 
most of the beds on Portage Lake, consist almost exclusively of varieties of non- 
quartziferous! felsitic porphyry; two kinds predominate; one of these has a chocolate- 
brown to liver-brown, suberystalline to compact almost vitreous matrix containing 
very scattered minute crystals of triclinic feldspar of the same color as the base. 
The other and rarer variety, also non-quartziferous, has a chocolate-brown, compact to 
minutely crystalline matrix, in which lie erystals, $ to-$inch long, of a flesh-colored 
triclinic feldspar. In some beds there appear pebbles of a flesh-red rock,? composed 
almost entirely of granular feldspar, containing small specks of a black undetermined 
mineral. In some instances the feldspar is wholly triclinic, in others the twin-striation 
is frequently absent. This variety of pebble is altogether absent in some beds, at least 
where they are opened, while in others they predominate, as in the Albany and Boston 
Conglomerate. Pebbles of compact melaphyr® and of melaphyr-amygdaloid* also 
occur, but are quite subordinate in number to those already enumerated. The normal 
form of cement is a fine-grained sandstone, composed apparently of the same material 
as the pebbles. Often the cement is very subordinate in volume, the pebbles touching 
each other. Frequently, however, the reverse is the case, and often, the sandstone 
forms layers from less than an inch to many feet in thickness.® 
The large dump at the old shaft of the Albany and Boston mine is a good 
place to study these conglomerates. The pebbles are usually large, some- 
times six inches ora foot in greatest diameter. They are commonly oblong, 
or flattened. A pink to brick-red, medium-grained, highly-crystalline augite- 
syenite predominates, but dark-red to chocolate-brown, aphanitic, felsitic 
porphyry is abundant. I have studied four typical samples of the peb- 
bles of this conglomerate, both macroscopically and in the thin section. 
Of theso the first is a finer-grained but distinctly crystalline, dark red- 
dish-brown augite-syenite, the predominant red color being thickly dotted 
with small black points. In the thin section the chief ingredient is seen 
to be oligoclase in quite good-sized and fresh crystals. Orthoclase 
follows next, and there is some quartz, most of which appears to be of a sec- 
ondary origin. The black particles seen macroscopically resolve themselves 
into open aggregations of brown ferrite and black opacite (magnetite and 
ferrite), commonly of very irregular outline, but not unfrequently having 
linear boundaries. From the same appearances seen in many others of 
1 Evidently meaning without visible porphyritie quartz. 
2Granilic porphyry or augite-syenite of this volume. 
3Diabase of this report. 
4Diabase-amygdaloid. 
5R. Pumpelly, in Geological Survey of Michigan, Vol. I, Part II, pp. 16, 17. 
